HD 17156 b

HD 17156 b

Exoplanet		List of exoplanets
300px Size comparison of HD 17156 b with Jupiter.
Parent star
Star		HD 17156
Constellation		Cassiopeia
Right ascension	(α)	02h 49m 44.49s
Declination	(δ)	+71° 45′ 11.6″
Apparent magnitude	(mV)	8.17
Distance		245 ± 13 ly (75 ± 4 pc)
Spectral type		G0
Mass	(m)	1.275 ± 0.018 M☉
Radius	(r)	1.5007 ± 0.0076 R☉
Temperature	(T)	6079 ± 80 K
Metallicity	[Fe/H]	0.24 ± 0.05
Age		3.38+0.20 −0.47 Gyr
Orbital elements
Semi-major axis	(a)	0.1589 AU
Eccentricity	(e)	0.6768 ± 0.0034
Orbital period	(P)	21.2163979 ± 0.0000159 d
Inclination	(i)	86.573 ± 0.060°
Argument of periastron	(ω)	121.71 ± 0.43°
Time of periastron	(T0)	2454438.4835±0.0025 JD
Semi-amplitude	(K)	274.2 ± 2.0 m/s
Physical characteristics
Mass	(m)	3.191 ± 0.033 MJ
Radius	(r)	1.0870 ± 0.0066 RJ
Density	(ρ)	3470 kg m−3
Surface gravity	(g)	67.0 ± 2.4 m/s²
Temperature	(T)	?
Discovery information
Discovery date		14 April 2007
Discoverer(s)		Fischer et al.
Discovery method		Radial velocity and Transit
Discovery site		W. M. Keck Observatory Subaru Telescope
Discovery status		Published
Database references
Extrasolar Planets Encyclopaedia		data
SIMBAD		data
Exoplanet Archive		data
Open Exoplanet Catalogue		data

HD 17156 b is an extrasolar planet approximately 240 light-years away in the constellation of Cassiopeia. The planet was discovered orbiting the yellow subgiant star HD 17156 in April 2007. The planet is classified as a relatively cool hot Jupiter planet slightly smaller than Jupiter but slightly larger than Saturn in a so-called "torched orbit". This highly-eccentric three-week orbit takes it approximately 0.0523 AU of the star at periastron before swinging out to approximately 0.2665 AU at apastron. Its eccentricity is about the same as 16 Cygni Bb, a so-called "eccentric Jupiter". Until 2009, HD 17156 b was the transiting planet with the longest orbital period.

To-scale diagram of the planet's orbit. The gray line indicates the transit window.

The planet was discovered on April 14, 2007 by a team using the radial velocity method on the Keck and Subaru telescopes. The team made an initial, negative, transit search, but they were only able to cover 25% of the search space. This left the possibility of a transit open.

After the possibility of a transit was discussed on oklo.org, various groups performed a follow-on search. These searches confirmed a three-hour transit on October 2, 2007 and a paper was published two days later.

Careful radial velocity measurements have made it possible to detect the Rossiter–McLaughlin effect, the shifting in photospheric spectral lines caused by the planet occulting a part of the rotating stellar surface. This effect allows the measurement of the angle between the planet's orbital plane and the equatorial plane of the star. This planet's spin-orbit angle was initially measured by Narita in 2007 as +62 ± 25 but has been remeasured by Cochran +9.4 ± 9.3 degrees. By analogy with HD 149026 b, a low angle would imply the formation of the planet was peaceful and probably involved interactions with the protoplanetary disc. A much larger angle suggests a violent interplay with other protoplanets. Given its eccentricity the latter is equally probable until the discrepancy of the Rossiter results is resolved.

It is so distant from its star and eccentric, that it will never enter a secondary eclipse relative to Earth, of the planet behind the star. The star's true temperature, and so the planet's surface temperature, cannot be measured with accuracy. Due to the high eccentricity the atmosphere of the planet undergoes a factor of 27 variation in stellar flux during each orbit.

See also

• 16 Cygni Bb
• HD 17156 c

External links

Media related to at Wikimedia Commons